Medical ultrasound imaging systems have been designed under the premise that the impinging ultrasonic energy produces a linear response. In order to be linear, a system must obey the following equation: IF
x1(t)=a system input and y1(t) is the corresponding output
and
x2(t)=a system input and y2(t) is the corresponding output THEN
(a*x1(t)+b*x2(t)) as an input gives (a*y1(t) and b*y2(t)) as an output
Current trends in research concentrate on investigating non-linear responses to ultrasonic energy. For example, some contrast agents have been found to provide a second harmonic response to impinging ultrasound energy, and this response can be used to provide increased diagnostic information about the surrounding tissues. A second harmonic response occurs when an agent under increasing ultrasonic pressure, "maps" energy into its harmonics, instead of the fundamental.
In one prior art diagnostic system, disclosed by Johnson, et al in U.S. Pat. No. 5,456,257, the presence of coated microbubble contrast agents in the body of a patient is detected by transmitting ultrasonic energy which causes the destruction of the microbubble. The diagnostic system detects the microbubble destruction through phase insensitive detection and differentiation of echoes received from two consecutive ultrasonics transmission. The destruction of a microbubble may also be used as a point source of acoustic energy for aberration correction, whereby the timing of the beamformer is adjusted from an analysis of beamformer signals resulting from a detected microbubble destruction event.
It would be desirable to be able to enhance the detection of microbubbles in the bloodstream relative to the tissue.